Control power sources for an OA equipment, a solar power generator system, an automobile, and an uninterruptable power supply, etc., use a choke coil as an electronic device, and such a choke coil uses a ferrite magnetic core or a dust core. The ferrite magnetic core has a disadvantage that the saturated magnetic flux density is small. Conversely, the dust core formed by molding metallic powders has a higher saturated magnetic flux density than that of soft magnetic ferrite, and has a good DC superposition characteristic.
The dust core is demanded to have a magnetic characteristic capable of obtaining a large magnetic flux density by a small applied magnetic field and a magnetic characteristic of a small energy loss inherent to a change in the magnetic flux density because of the demands for improvement of the energy exchange efficiency and reduction of the generated heat. The energy loss includes a core loss (iron loss) caused when the dust core is used in an AC magnetic field. The core loss (Pc) can be expressed as a sum of a hysteresis loss (Ph) and an eddy current loss (Pe) as is indicated in the following Equation (1). The hysteresis loss is proportional to an operating frequency as is indicated in the following Equation (2), and the eddy current loss (Pe) is proportional to the square of the operating frequency. Hence, the hysteresis loss (Ph) is dominating at a low-frequency range, while the eddy current loss (Pe) is dominant at a high-frequency range. The dust core needs a magnetic characteristic that reduces the occurrence of such a core loss (Pc).Pc=Ph+Pe  (1)Ph=Kh×f Pe=Ke×f2  (2)
where Kh is a hysteresis loss coefficient, Ke is an eddy current loss coefficient, and f is a frequency.
In order to reduce the hysteresis loss (Ph) of the dust core, it is necessary to facilitate the mobility of a magnetic domain wall, and in order to do so, it is appropriate to reduce the magnetic coercive force of the soft magnetic powder particles. By reducing the magnetic coercive force, both improvement of the initial permeability and reduction of the hysteresis loss can be achieved. The eddy current loss is inverse proportional to the specific resistance of the core as is indicated in the following Equation (3).Ke=k1Bm2t2/ρ  (3)
where k1 is a coefficient, Bm is a magnetic flux density, t is a particle size (thickness of a plate material), and ρ is a specific resistance.
Such a dust core is used for a switching power supply, etc., for electronic devices, and is used as a core of a reactor that eliminates AC components (noises) superimposed on a DC output. In order to accomplish the eliminating effect of noises, the dust core used as the core of a reactor needs to have a high saturated magnetic flux density. Moreover, since main currents of a power supply device flow through the reactor, if the loss of the dust core is large, a large amount of heats is generated. In order to prevent such heat generation, it is necessary for the dust core used as the core of the reactor to have a low core loss.
Hence, as shown in FIG. 13, in order to increase the current value for saturating the magnetic core, prevent a saturation of the magnetic flux density even if a large current flows, and ensure the function as the magnetic core of the reactor, a technique is known which forms a plurality of gaps orthogonal to the magnetic path of the dust core that is the core of a reactor, and disposes an insulative (non-magnetic) material formed of, for example, a resin in such gaps (see, for example, Patent Literatures 1 to 3).
According to the technologies disclosed in Patent Literatures 1 to 3, however, the leakage flux near the gap causes the winding and the core to generate heat, and when such technologies are applied to a reactor, the circuit efficiency decreases. Moreover, the leakage flux becomes a noise source to a peripheral device, and induces an eddy current loss to a peripheral conductor. Furthermore, from the standpoint of the structure, an assembling process of the core becomes complex, resulting in the cost increase, and a gap and a magnetic material collide with each other and move apart from each other at each gap, resulting in a cause of undesired sound at the time of actuation.
Hence, in order to address or reduce various technical issues due to a gap when such a gap is provided in the magnetic core of a reactor, a reactor is known which uses, as the magnetic core of the reactor, a nanocrystal material that is a low permeability material and which eliminates a gap (see, for example, Patent Literatures 4 and 5).
Patent Literature 1: JP 2004-095935 A                Patent Literature 2: JP 2007-012866 A        Patent Literature 3: JP 2009-224584 A        Patent Literature 4: JP 2006-344867 A        Patent Literature 5: JP 2006-344868 A        
According to the dust core formed of the nanocrystal material used in Patent Literatures 4 and 5, however, the powder itself is rigid, molding is difficult and the density of the dust core becomes low (equal to or less than 85% of a theoretical density). Hence, the permeability of the dust core formed of the nanocrystal material can be low, but the permeability/DC superposition characteristic becomes poor. Moreover, the maximum magnetic flux density of the material itself is small, even if it is used as a reactor, an L value (an inductance) largely decreases at a high magnetic field.
The present invention has been made to address the above-explained problems, and it is an object of the present invention to provide a reactor and a method for manufacturing the same which use, as a magnetic core of the reactor, a dust core formed by a high-pressure molding while uniformly dispersing insulative fine powders around a soft magnetic powder to maintain a high density, and the dust core with a low permeability to improve the DC superposition characteristic of the magnetic core of the reactor, thereby eliminating a gap and downsizing of the reactor.